A practical application of total internal reflection is found in

1. Analyze the question

The question asks for a practical application of total internal reflection (TIR). We need to identify which of the given options relies on this specific optical phenomenon to function.

2. Explain the concept

Total Internal Reflection (TIR) occurs when light traveling from a denser medium to a less dense medium hits the boundary at an angle greater than the critical angle. Instead of refracting (passing through), the light is completely reflected back into the denser medium.

For TIR to occur, two conditions must be met:

1. Light must travel from a medium with a higher refractive index to one with a lower refractive index.
2. The angle of incidence must be greater than the critical angle for the pair of media.

3. Show the solution

Let's evaluate the options:

• A. Rainbow: This is primarily caused by the refraction, dispersion, and internal reflection of sunlight within water droplets. While internal reflection occurs, it is not the primary mechanism defined as "total internal reflection" in the context of fiber optics.
• B & C. Diverging and Converging Lenses: These operate based on the principle of refraction. Light passes through the glass, bending according to Snell's Law to focus or spread light.
• D. Fiber Optics: This is the classic application of TIR. A fiber optic cable consists of a core with a high refractive index surrounded by a cladding with a lower refractive index. Light signals enter the core and strike the boundary between the core and cladding at an angle greater than the critical angle. Consequently, the light is trapped inside the core and "bounces" along the length of the cable with minimal loss of signal.

4. Identify the answer

Based on the principles of optics, fiber optics is the most direct and well-known practical application of total internal reflection.

Correct Option: D

5. Exam Tip

To remember the conditions for TIR, use the mnemonic "D-to-R, Angle-is-Greater":

• D-to-R: Light must travel from a Denser medium to a Rarer medium.
• Angle-is-Greater: The angle of incidence must be Greater than the critical angle.